This invention relates to an optical recording apparatus such as an optical printer whose light source is a light emitter array, and more particularly to a diagnostic apparatus for diagnosing whether or not light emitter elements forming the light emitter array in the optical printer are satisfactorily or normally emitting light.
A conventional optical printer having a light emitting diode (LED) array as its light source has a structure as schematically shown in FIG. 10. Referring to FIG. 10, data to be recorded is supplied from a host computer 100 to an LED array printer 200. This LED array printer 200 is generally composed of a driver circuit 201, an LED array 202, an image focusing lines array 203 and a photoconductive drum 204. The data is supplied in a digital form so as to selectively cause emission of light from corresponding LED elements (not shown) in the LED array 202. In this case, data corresponding to one line is sequentially supplied from the host computer 100 to cover all of the LED elements arrayed to form the LED array 202. The data supplied from the host computer 100 is subjected to serial-parallel conversion in the driver circuit 201 so as to selectively cause emission of light from the LED elements in the LED array 202 according to the data supplied from the host computer 100. Light emitted from the energized LED elements among those forming the LED array 202 is focused by the focusing lens array 203 to form a dot image on the photoconductive drum 204. Such a manner of line sequential scanning for causing emission of light from selected LED elements is continued so as to sequentially form a dot image on the photoconductive drum 204 being rotated. Thus, character, pattern or like images are recorded on the photoconductive drum 204. The dot images formed on the photoconductive drum 204 are then transfer printed on a sheet of paper by a method such as an electrostatic recording method.
When the luminance of any one or more of the LED elements forming the LED array 202 is subject to a variation, it leads to the problem that the optical density of the recorded image is not maintained constant, and the image quality will be greatly impaired or degraded. Such a variation in the luminance of emission is attributable to various factors including the temperature, corruption and secular variation. An attempt to deal with such a problem is disclosed in, for example, JP-A-61-264361 which discloses that the quantity of light emitted from an LED array is detected by a luminous power sensor, and the period of time of emission from the LED array is controlled on the basis of the result of the luminous power detection so as to maintain constant the quantity of light emitted from the LED array. On the other hand, JP-A-62-270350 and JP-A-63-25066 disclose a method for deciding whether an LED element is normal or not. According to the disclosures of these two patent applications, a resistor is connected in series with an LED element to be inspected, and this LED element is decided to be normal by detecting current which flows through the resistor in response to the energization of this LED element.
However, JP-A-61-264361 cited above does not refer to the case where any one of the LED elements in the LED array does not emit light due to, for example, disconnection of its power supply lead and does not also refer to the detection of the quantity of light emitted from each of the LED elements.
On the other hand, when any one of the LED elements becomes faulty, the corresponding portion of the dot image drops out. In such a case, the problem is more serious than the case of a non-uniform image density distribution in that the information will not be sometimes correctly recorded. Therefore, it is necessary to diagnose whether or not any one of the LED elements in the LED array becomes faulty in the state in which the LED array is incorporated in a printer. JP-A-62-270350 and JP-A-63-25066 cited above meet such a demand. However, it is impractical to connect one resistor in series with each of the many LED elements in the LED array printer. Although employment of a switching means may be preferable for decreasing the number of the series resistors, this method is also impractical in that the structure of the switching means becomes complex in itself.